The major thrust of this study is to elucidate the molecular basis, with particular emphasis on the genetic aspects, of neoplastic transformation of normal tissues. Two experimental model systems were used in this project: (1) Intracisternal A particle (IAP) containing neoplasms, a BALB/c mammary tumor cell line, its hybrid and cybrid clones, a plasma cell tumor (MCPC104E), and feral mouse cell lines of both Mus musculus and Mus cervicolor; and (2) two rat leukemia (helper) viruses, a) KSV (RHHV), a transforming rat virus isolated in this laboratory and b) WR-RaLV, a wild rat tumor virus isolated from a wild rat carcinoma induced by 3-methylcholanthrene. Multidisciplinary approaches were required in this investigation. They involve nucleic acid and protein chemistry, tissue culture, cytogenetics, immunology and tumor biology. Since we have completed extensive characterizations morphologically, biologically, biochemically and immunologically of these retroviruses, our current emphasis of research efforts focuses on: (1) integrations of horizontally transferred viral genome and its localization in the host cell; (2) regulation of viral gene expressions; (3) integration site(s) for exogenous viral DNA sequences and (4) mapping of a viral genome.